1. Field of the Invention
The present disclosure is generally related to a noise isolation technique useful in high-speed digital systems on packages and printed circuit boards (PCBs), collectively referred to herein as a component carrier, and more particularly is related to electromagnetic bandgap structures that provide ultimate noise isolation (i.e., from DC to a substantially infinite frequency).
2. Description of the Related Art
Over the last decade, the scaling of the CMOS transistors has enabled the design of microprocessors operating at multi-gigahertz frequencies. This trend, based on the International Technology Roadmap for Semiconductors (ITRS), is expected to continue over many years for both desktop and mobile computers. Along with the scaling of the transistor, the number of transistors on a chip is doubling every 18 months, based on Moore's law. In addition, long-haul communication bandwidth is estimated to be doubling every nine months, much faster than Moore's law. A combination of voltage scaling and Moore's law is causing an alarming increase in the power consumed by microprocessors. Since computers are broadband systems, the current needs to be supplied to the switching circuits over a broad frequency range from DC to at least the second harmonic of the clock frequency. This trend in microprocessors is causing a major challenge for distributing power in computer systems. With voltage scaling and wireless integration in mobile computers, the tolerance to power supply noise is rapidly decreasing.
A major contribution to power supply noise comes from the package and board level interconnections. Because of their distributed electrical characteristics, package and board interconnections can support electromagnetic waves in the power distribution network (PDN). One of the most important areas in high-speed digital systems is the design and analysis of the PDN. The power distribution network supplies power to core logic and I/O circuits in any digital systems. As clock speeds increase, and signal rise time and supply voltages decrease, the transient currents injected into the power distribution planes can induce voltage fluctuation on the power distribution network. This undesired voltage fluctuation on the power/ground planes is commonly known as simultaneous switching noise (SSN), power supply noise, and delta-I noise. Power supply noise leads to unwanted effects on PDN such as ground bounce, false triggering in digital circuits, and waveform distortion in the time domain. It has been shown (in references [1]-[3] identified below) that power supply noise induced by a large number of simultaneously switching circuits in a printed circuit board (PCB) or multichip module (MCM) can limit the performance of the system ([1] R. R. Tummala, E. J. Rymaszwski, and A. G. Klopfenstein, Microelectronics Packaging Handbook, 2nd ed., New York: Chapman & Hall, 1997, pt. I; [2] R. R. Tummala, Fundamentals of Microsystems Packaging, McGraw-Hill, 2001; [3] S. Hall, G. Hall, and J. A. McCall, High-Speed Digital System Design, John Wiley & Sons, Inc., 2000). Especially, power supply noise can be transferred to anywhere in power/ground planes in packages and boards since power/ground planes behave as a parallel-plate waveguide at high frequencies.
Power/ground planes in packages and PCBs represent large metal layers separated by a small dielectric distance. Due to the small dielectric distance, power/ground planes in the package and PCB are capacitive at low frequencies and are therefore ideal for supplying power to the integrated circuits. However, with increase in frequency, planes become inductive and resonate at discrete frequencies. Conventional power and ground planes have a dielectric thickness of less than 100 mils and the dielectric thickness becoming thinner with advances in technology. The lowest transverse magnetic (TM) and transverse electric (TE) modes for the parallel-plate waveguide have cut-off frequencies in the order of hundreds of gigahertz, which implies TM and TE modes of the parallel-plate waveguide are not a major concern for the systems operating at 10 GHz and below. Therefore, the only modes of concerns are the transverse electromagnetic (TEM) modes of the parallel-plate waveguide and cavity resonator modes due to the finite size of the power/ground planes. For these modes for the parallel-plate waveguide, it was assumed that the conducting planes have infinite length in the x and z directions. However, real power and ground planes have the finite size of the width and length, which means that waves propagating to the edges of the power/ground planes have to be reflected back and forth. The rectangular cavity resonator modes occur at the following frequencies
                              f          cavity                =                              1                          2              ⁢              π              ⁢                              μɛ                                              ⁢                                                                      (                                                            m                      ⁢                                                                                          ⁢                      π                                        a                                    )                                2                            +                                                (                                                            n                      ⁢                                                                                          ⁢                      π                                        b                                    )                                2                            +                                                (                                                            p                      ⁢                                                                                          ⁢                      π                                        d                                    )                                2                                                                        (        1        )            where μ is the permeability of a dielectric material, ∈ is the permittivity of a dielectric material, and m, n, and p are mode numbers equal to 0, 1, 2, . . . , but except m=n=p=0 and a is the width of the power/ground planes, b is the length of the power/ground planes, d is the dielectric thickness of a dielectric layer in the power/ground planes. But, in practical power/ground planes, a dielectric thickness d is much smaller than both the width (a) and the length (b), which means the standing wave patterns along dimension d will be at frequencies that are tens to hundreds of times higher than the resonant frequencies of waves along the width and length of power/ground planes. Hence, the cavity resonant frequencies in equation (1) for practical power/ground planes can be written as:
                              f          cavity                =                              1                          2              ⁢              π              ⁢                              μɛ                                              ⁢                                                                      (                                                            m                      ⁢                                                                                          ⁢                      π                                        a                                    )                                2                            +                                                (                                                            n                      ⁢                                                                                          ⁢                      π                                        b                                    )                                2                                                                        (        2        )            
One typical approach to isolate digital circuits from other digital circuits on packages and printed circuit boards is to split the power plane for both power and ground planes. The gap in power plane or ground plane can partially block the propagation of electromagnetic waves. For this reason, split planes are usually used to isolate digital circuits from other digital circuits. Although split planes can block the propagation of electromagnetic waves, part of the electromagnetic energy can still couple through the gap. Hence, this method only provides a marginal isolation (−20 dB˜−60 dB) at high frequencies (usually above ˜1 GHz) and could create a serious problem as the sensitivity of digital circuits increases and the operating frequency of the system increases. Generally, split planes provide good isolation (−70 dB˜−80 dB) at low frequencies (usually below ˜1 GHz) but show poor isolation (−20 dB˜−60 dB) at high frequencies because of electromagnetic coupling. Especially, noise at resonance peaks can be transferred easily from one place to the other in split planes since the isolation level at these resonance peaks is around −20 dB.
Electromagnetic bandgap (EBG) structures have become popular because of their ability for suppressing unwanted electromagnetic mode transmission and radiation in microwave and millimeter waves. The EBG structures are periodic structures in which the propagation of electromagnetic waves is forbidden in certain frequency bands. In these EBG structures, the constructive and destructive interference of electromagnetic waves results in transmission and reflection bands. A common feature of periodic structures is the existence of frequency bands where electromagnetic waves are highly attenuating and do not propagate. Among these EBG structures, alternating impedance electromagnetic bandgap (AI-EBG) structure was developed for noise isolation in mixed-signal systems in 2004 and showed excellent isolation (−80 dB˜−140 dB), which is the best isolation reported. However, this type of AI-EBG structure only provides excellent isolation in high frequency range but does not provide excellent isolation in DC and low and mid frequency ranges. In high-speed digital systems, good noise isolation is required from DC to at least the second harmonic frequency of the clock frequency. These days the clock frequency of the high-speed digital systems is going up, and will continue to increase in the future.
Therefore, the development of a better noise isolation method is needed for good performance of a high-speed digital system on packages and printed circuit boards (PCBs). It would thus be desirable to provide an improved noise isolation structure with improved operating characteristics while maintaining ease of manufacturability.